As illustrated In FIG. 29 to FIG. 31, a prior art line filter has a structure wherein a winding bobbin 53 prepared by molding a resin material into two-piece construction is assembled on one of the magnetic legs of a square shaped closed magnetic circuit core 52 made of a high permeability magnetic material such as ferrite and the like and a pair of windings 54 are wound around the wire winding bobbin 53 by rotating the winding bobbin 53 by means of a gear formed on the end surface or on the central part of the winding bobbin 53.
Next, metal terminals 55 are placed in insertion holes arranged on flanges of the winding bobbin 53 and the windings' start and end lead wires are wrapped around and securely connected by soldering to the metal terminals 55. Further, a core cover 56 is installed so as to cover the core 52 and fixed on to the core 52 by having its opening's surfaces inserted in slots 57 formed on the outer surfaces of the flanges situated on both ends of the winding bobbin 53.
According to the prior art structure as described in the above, the square shaped closed magnetic circuit core 52 was not so securely fixed to the winding bobbin 53 and there was a danger that the core cover 56 might be detached because the fitting between the winding bobbin 53 and the core cover 56 was not good enough, resulting in an insufficient fixing between the square shaped closed magnetic circuit core 52 and the winding bobbin 53.
Particularly, there was a problem of magnetic flux leakage caused by the square shaped closed magnetic circuit core 52, inserted in the through hole of the winding bobbin 53, wobbling around the periphery of the winding bobbin 53 with the through hole serving as the center axis, thereby presenting magnetic disturbances to other components or allowing noises to infiltrate into the square shaped closed magnetic circuit core 52.